There is a considerable need for dispatch communications, i.e., simplex bi-directional communications between a dispatcher and remote (field) units. This need is conventionally filled by specialized equipment operating over dedicated frequencies. Examples of this type of equipment are the dispatch radios used by police, fire, ambulance, taxi, and delivery services. In dispatch systems, a single dispatch unit typically uses one frequency (frequency “A”) for transmission and another frequency (frequency “B” for reception, with all field units using frequency “B” for transmission and frequency “A” for reception.
Dispatch radios share many problems with other simplex systems, e.g., construction-site walkie-talkie radios, personal-service radios, and other business radios. In such systems, all units typically use a single frequency for both transmission and reception. By necessity, the number of units in such systems is severely limited.
Such communication systems are often simplex. That is, a given unit may only transmit or receive at one time, but not both. This limitation is both a weakness and a strength of such systems. Since only one unit of a communicating pair may be transmitting at one time, interruptions are impossible, regardless of the urgency involved. On the other hand, the equipment need not have the complexity and expense of full duplex communication equipment. Because of their similarities, dispatching and single-frequency systems may be generally classed as push-to-talk (PTT) systems for the purposes of this discussion.
PTT systems suffer from a significant number of problems. A major one of these problems is that PTT systems are typically proprietary. That is, the equipment for a given system is often made by a single manufacturer. This obliges the user/owner to deal with this single manufacturer. The equipment is therefore often more expensive than similar equipment for other services, even though that other equipment may be more sophisticated than the needed equipment. The reasons for this are complex, involving the scale of production as well as the lack of competition.
Similarly, such equipment often must be serviced by specially trained and licensed personnel. Again, being a small market, a given area will often have only a small pool of qualified service agencies/personnel. Such an agency is typically licensed or certified by the manufacturer. This again leaves the user/owner at the mercy of the manufacturer through the service personnel, resulting in a decrease in competition and an increase in service expenses.
Because such PTT equipment is often manufactured and serviced by a single company, the user/owner may well be left without support of any kind should that manufacturer cease to do business. Alternatively, the user/owner of the equipment may be faced with a considerable difficulty should the local service agency of the equipment manager cease to represent that manufacturer. This often necessitates that the equipment be returned to the manufacturer for servicing, thereby effecting unreasonable delays.
PTT systems are typically manufactured to fulfill specific and unique requirements. That is, while the PTT dispatch system used by a taxicab company is similar in design and function to that used by a fire department, they are designed to operate at different frequencies and are not interchangeable. This non-interchangeability extends beyond physical constraints and into the areas of licensing and legislation. Therefore, a small rural volunteer fire department on a tight budget is constrained from using donated taxi dispatching systems. The systems and their components are not interchangeable.
Because of this incompatibility of hardware and operating frequencies, two different PTT systems cannot readily intercommunicate. For example, in an emergency situation it may be desirable to coordinate police, fire, and medical field units from a single dispatching unit. This is not normally feasible without a special cross-service dispatching unit and/or multiple dispatching units in the same location. Overcoming such incompatibilities increases the expense of each of the systems while being an inefficient compromise at best. Additionally, the use of such a centralized and complex dispatching center often necessitates the use of a highly skilled and specially trained dispatcher (operator). This, too, increases system expense.
PTT systems typically operate within specific frequency bands by law. These bands have limited capabilities, thus creating a problem when many services must use the same band. Since each PTT system providing a given class of service, e.g., taxicab dispatching, must share the same band while simultaneously utilizing different channels (frequency allocations with the band), such channels are often at a premium in large metropolitan areas. Occupation of all available channels in a given area would prohibit the assignment of another channel in that area. Therefore, a potential new user may be inhibited from receiving a needed license.
Likewise, since a shortage of channels may produce a waiting list for licenses, the loss of a license for a given channel, however briefly and for whatever reason, may result in the assignment of that specific channel to a new licensee, thereby effectively driving the former license holder out of business.
PTT systems also have coverage problems. Not only does the specific equipment have an operating range limited by design, the operating range is also limited by geography. For example, operation is typically limited to “line-of-sight” for the frequencies and signals involved. Shadows may thereby be cast by natural and artificial geography. In a typical scenario, for example, a taxicab dispatching service may lose contact with any cab in an area shadowed by a hill. Similarly, a messenger service may have only intermittent and unpredictable contact with messengers in a downtown area due to a large number of steel and concrete buildings. Both problems derive from the very structure of a PTT dispatching system. That is, all mobile field units must communicate with a fixed dispatching unit via an electromagnetic line-of-sight. Therefore, if the geophysical relationship between the field unit and the dispatching unit is such as to inhibit transmission and/or reception, then communication is lost.
Dispatching systems make up a significant portion of PTT systems in use. PTT dispatching systems typically have a single dispatching unit and a plurality of field units. As previously mentioned, the dispatching unit may transmit on frequency “A” and receive on frequency “B,” while the field units transmit on frequency “B” and receive on frequency “A.” This means that a PTT dispatching system has an assigned dispatching unit that differs in kind as well as operation from the field units.
The centralized dispatching unit of a PTT dispatch system typically transmits to all field units simultaneously. That is, a typical two-frequency PTT dispatching system cannot readily communicate to only a subset of the assigned field units. There are systems in which selective dispatching is implemented, but all such systems are expensive and inefficient. For example, each field unit may have an address affixed to the beginning of each dispatch intended exclusively therefore. The use of such an address header therefore allows private messages to be dispatched. However, this increases radically in complexity when multiple (but not all) field units are to be addressed.
In an alternative dispatching scheme, the centralized dispatching unit may have multiple transmission frequencies. This allows normal dispatches (i.e., those intended for all field units) to be transmitted on a first frequency with selective dispatches being transmitted on a second frequency. In this scheme, the dispatcher would instruct the appropriate field units to switch to the second frequency prior to the transmission of a selective dispatch. However, this scheme requires an increase in complexity in both the dispatch and field units, including the incorporation of a switching mechanism with a corresponding decrease in reliability.
The complexity of dispatching to selected units using known conventional dispatching schemes increases dramatically when the number and addresses of the selected units is dynamic. In a highly dynamic emergency situation, for example a forest fire, the “groups” to be addressed may change many times in the course of the emergency as personnel move from one location to another. Conventional dispatching systems simply lack the flexibility to change fast enough to optimize the dispatching. Rather, under most such dynamic situations, the dispatcher is reduced to general all-unit dispatching only.
There are many circumstances when general all-unit dispatches are less than optimal. For example, peace officers may be making a covert entry into a building. The last thing desired in such a situation is a sudden outburst over the radio. Selective dispatching, therefore, should not only be capable of easily and efficiently dispatching to only selected field units, it should be capable of easily and efficiently not dispatching to selected field units. This is not easily accomplished with currently available PTT dispatching systems.
Another problem exists with conventional PTT dispatching systems in that multi-level dispatching is not practical without exceptionally complex equipment and/or operations. In a multi-level dispatching scheme of four levels (e.g., headquarters, groups, teams, field units), an overall dispatcher at headquarters would be capable of dispatching down directly to all field units and/or to all group dispatches. Each group dispatcher would be capable of dispatching down to all field units within that group, down to all team dispatchers within that group, and/or up to the headquarters dispatcher. Each group dispatcher would be capable of dispatching down to all field units within that team, up to the group dispatcher for that team, and/or (optionally) up to the headquarters dispatcher. Such a “chain of command” structure is ideal for coordination during major emergencies (such as earthquakes or floods), but cannot be readily realized with conventional PTT dispatching services without the complexity and expense of military-type equipment.
The dispatching unit of a PTT system is different in kind to the field units. The dispatching unit is typically a fixed “base station.” As such, the dispatching unit is tied to mains service and is not mobile. This causes PTT dispatching systems to be severely handicapped during fluid situations where the base station may be lost. To cover for such circumstances, a “mobile base unit” may be used, typically an alternative base station mounted in a truck or other vehicle. Such a mobile base station adds significantly to the overall expense of a PTT system. The expense involved often drives such a feature beyond the range of small communities who, ironically, may best benefit from it.
Again, because the dispatch unit of a PTT dispatch system is inherently different than a field unit, a field unit cannot normally be used as an alternative dispatch unit in the event of failure of the dispatch unit. Therefore, the integrity of the entire system depends upon the integrity of a single dispatch unit. Should the dispatch unit fail, the entire system fails. This poses a less-than-optimal situation when the PTT dispatch system is critical, necessitating the acquisition of a second dispatch unit whose sole function is to stand by in case the primary dispatch unit should fail. Again, this represents a waste of resources.
Where the PTT dispatch system is less critical, the failure of the dispatch unit causes the system to be inoperative while the dispatch unit is repaired or replaced. This necessitates the use of alternative communications (e.g., telephones), which provide an awkward solution at best.
The field units in some PTT dispatch systems do not normally have the ability to intercommunicate. That is, the field units in a system normally all transmit on frequency “B” and receive on frequency “A.” No field unit can then receive the transmission from another field unit. This lack of intercommunication necessitates that a typical field unit may convey information to another field unit only through the dispatch unit. This places an additional burden upon the dispatcher and slows down the conveyance of intelligence, making coordinated efforts more difficult.
Certain types of specialized field units have the ability to transmit and receive upon alternative frequencies. When this ability is engaged, those specific field units effectively are removed from the PTT dispatch system and become a local single-frequency PTT system. This condition poses the potential of a serious problem during a crisis situation. While the needed and necessary local intercommunication is enabled, those field units are inhibited from receiving information from the dispatch unit. Such information may be critical e.g., the inability of expected backup to arrive when planned.
Another problem exists with conventional PTT dispatching system in that, other than by direct query and extrapolation therefrom, the dispatcher has no way of knowing the locations of the field units. This means that, even if sophisticated multi-channel equipment is used, the dispatch unit cannot readily transmit a zone dispatch, i.e., a dispatch to all units within a specific geographical area. During a crisis, considerable effort is expended for the sole purpose of keeping track of the individual field units. This effort often entails several people and a considerable amount of traffic for location determination. Such an ability, totally lacking in conventional PTT dispatching systems, would be invaluable coordinating even a small crisis (e.g., the coordination of taxicabs with the near-simultaneous arrival and departure of several major flights during a rush hour).
Conventional PTT dispatching systems often lack in system security. Such systems typically use conventional amplitude or frequency modulation (AM or FM) utilizing analog (i.e., non-digital) modulation techniques. This approach, while cost-effective, is very insecure and does little to inhibit eavesdropping.
A courier service, for example, depends heavily upon its established customer base for survival. Were an unscrupulous competitor to eavesdrop upon the courier service's dispatches for a relatively short period of time, that competitor might then be in a position to determine the courier service's major clients and the number of pick-ups and deliveries per week. With this information, the competitor may be able to successfully underbid the courier service for those clients.
In a similar but more critical vein, were an unscrupulous press able to monitor police dispatches during a major crisis, important information may be leaked that would jeopardize negotiations and perhaps cost lives.
One answer to the eavesdropping problem is to encrypt the information. This is a straightforward procedure in digital systems, but somewhat cumbersome and expensive in analog systems. While encoding can be successfully used in critical PTT dispatching services (police, fire, etc.), it is often cost-prohibitive for business systems.
Attempts to substitute for encryption often involve the use of elaborate codes. Such codes may require considerable training, hence expense, and are far from foolproof. A single disgruntled employee or lost/stolen codebook is all that is needed to compromise such a code.
In addition, a fundamental failing of conventional PTT systems is an inability to interface with the outside world. This lack of interface means an inability to place a telephone call through the system without involving the dispatcher. This type of situation may arise, for example, should an individual field employee (an employee with a field unit) be awaiting the results of a medical test for him/herself or a family member. The employee is faced with three choices: have the doctor/lab contact him/her through the system (in violation of individual privacy rights); stop and call the doctor/lab repetitively from a telephone until the results are available (inconvenient to both the employee and the employer; or stay at home until the results are available (even more inconvenient and a loss of income to both the employee and the employer.
Associated with this lack of outside-world interface is the inability to summon emergency services when seconds may count. This inability may directly endanger lives and/or property.
With the proliferation of cellular telephone service, the replacement of PTT systems with cellular telephone systems is now possible. Unfortunately, the use of standard cellular telephone systems in lieu of PTT systems is not easily accomplished.
The first problem encountered when replacing a PTT system with a cellular telephone system is that of overkill. The replacement of a simplex communication system with a full-duplex system represents a significant waste of resources. Not only must adequate bandwidth for full duplex communication be allocated, it often must be allocated for the full duration of the conversation, i.e., from the time the connection is made until the parties hang up. These inefficiencies are a result of the circuit-switching services of cellular telephony, and are directly translatable into fiscal losses.
Additionally, the call time for a cellular telephone service is significantly greater than that of a PTT service for a given message. Again, this is due to the active set up time needed for each call, and also for the fact that a cell phone's transmitter is must occasionally transmit even when the phone is only receiving. This excess of transmission leads to a shorter battery life than desired.
Another problem is that, since a cellular telephone system is capable of calling any other telephone anywhere in the world, it uses a dialing scheme essentially the same as the traditional wire-based telephone system. Therefore, even with one-button dialing, there is a considerable time between the commencement of dialing and the completion of the connection so that communication may occur. This delay, while small for any single call, quickly becomes unmanageable when the standard cellular system is used as a PTT dispatching system replacement.
What is needed therefore, is a system that is broad in functionality, is wide in area of coverage, is easily accessible, is pervasive, requires no special licenses, requires no special equipment, is inexpensive to use, has the flexibility of the global cellular telephone system, and has the rapidity and ease of use of a conventional PTT dispatching system.